1.00 Lecture 9
Methods and Objects
Access
Reading for next time: Big Java: sections 7.6, 7.7
Storing weather for a city
• We’ll first show an example of storing
temperature and precipitation data for cities.
– Our classes have some methods with arguments
– We’ll examine the methods and arguments: they’ll do
what you expect
• We’ll then extend our weather example to have
each city store a weather information object that
collects the weather data in one place
– These classes also have methods with arguments
– We’ll examine these methods too. They also do what
you expect (but your expectations will have to be a bit
more sophisticated!)
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SimpleCity
public class SimpleCity {
private String name;
private double avgTemperature;
avgTemperature;
private double precipAmt;
precipAmt;
public SimpleCity(String
SimpleCity(String n, double a, double p) {
name= n;
avgTemperature=
avgTemperature= a;
precipAmt=
precipAmt= p;
}
public String getName()
getName() {
return name;
}
public double getAvgTemperature()
getAvgTemperature() {
return avgTemperature;
avgTemperature;
}
public void setAvgTemperature(double
setAvgTemperature(double t) {
avgTemperature=
avgTemperature= t;
}
public double getPrecipAmt()
getPrecipAmt() {
return precipAmt;
precipAmt;
} }
SimpleWeatherTest
public class SimpleWeatherTest {
public static void main(String[] args)
args) {
SimpleCity boston=
boston= new SimpleCity("Boston",
SimpleCity("Boston", 40.0, 0.0);
SimpleCity cambridge=
cambridge= new SimpleCity("Cambridge",
SimpleCity("Cambridge", 40.0, 0.0);
// Now revise the Boston weather, which was corrected
boston.setAvgTemperature(41.0);
System.out.println
System.out.println("Boston:
println("Boston: " + boston.
boston.getAvgTemperature());
getAvgTemperature());
System.out.println
System.out.println("Cambridge:
println("Cambridge: "+
cambridge.
cambridge.getAvgTemperature());
getAvgTemperature());
}
}
// What is the output of this program?
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Passing Arguments
SimpleWeatherTest
main(…){…
City boston= …
boston.setAvgTemp(41.0);
…
Communication only
via arg list,
return value
Argument 1
SimpleCity boston
public void setAvgTemp(double t)
{
// Method makes its own copy
// of argument t
avgTemperature= t;
}
Setting the
Cambridge
temperature
would be the
same
Method/Object Exercise
• We now change SimpleCity and
SimpleWeatherTest slightly
– We rename them City and WeatherTest
– We also introduce a simple Weather class
– We’ll look at them briefly on the next slides
3
Weather class
public class Weather {
private double avgTemperature;
avgTemperature;
private double precipAmt;
precipAmt;
public Weather(double a, double p) {
avgTemperature=
avgTemperature= a;
precipAmt=
precipAmt= p;
}
public void setAvgTemp(double
setAvgTemp(double t) {
avgTemperature=
avgTemperature= t;
}
public void setPrecipAmt(double
setPrecipAmt(double pr) {
precipAmt=
precipAmt= pr;
}
public String toString()
toString() {
return ("Temperature: "+avgTemperature
"+avgTemperature+
avgTemperature+
" ; Precipitation: "+precipAmt
"+precipAmt);
precipAmt);
}
}
City class
public class City {
private String name;
private Weather cityWeather;
cityWeather;
public City(String n, Weather c) {
name= n;
cityWeather=
cityWeather= c;
}
public String getName()
getName() {
return name;
}
public Weather getWeather()
getWeather() {
return cityWeather;
cityWeather;
}
}
4
WeatherTest
public class WeatherTest {
public static void main(String[] args)
args) {
Weather today= new Weather(40.0, 0.0);
City boston=
boston= new City("Boston", today);
City cambridge=
cambridge= new City("Cambridge", today);
// Now revise the Boston weather, which was corrected
Weather bostonToday=
bostonToday= boston.
boston.getWeather();
getWeather();
bostonToday.setAvgTemp(41.0);
System.out.println
boston.
getWeather());
System.out.println("Boston:
println("Boston: " + boston
.getWeather
());
System.out.println
println("Cambridge:
System.out.
println("Cambridge: "+ cambridge.
cambridge.getWeather());
getWeather());
}
}
Exercise- Weather classes
• Download Weather, City, WeatherTest
• Import them into Eclipse
• Before running them, think about what the
output will be
• Compile and run them
• Is the output what you expected?
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Objects As Arguments
today=
Weather today= new Weather(40.0, 0.0);
City boston=
boston= new City("Boston", today);
City cambridge=
cambridge= new City("Cambridge", today);
// Now revise the Boston weather, which was corrected
Weather bostonToday=
bostonToday= boston.
boston.getWeather();
getWeather();
bostonToday.setAvgTemp(41.0);
40.0 0.0
Weather
Objects As Arguments
today=
Weather today= new Weather(40.0, 0.0);
City boston=
boston= new City("Boston", today);
boston=
City cambridge=
cambridge= new City("Cambridge", today);
// Now revise the Boston weather, which was corrected
Weather bostonToday=
bostonToday= boston.
boston.getWeather();
getWeather();
bostonToday.setAvgTemp(41.0);
“Boston”
City
40.0 0.0
Weather
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Objects As Arguments
today=
Weather today= new Weather(40.0, 0.0);
City boston=
boston= new City("Boston", today);
boston=
City cambridge=
cambridge= new City("Cambridge", today);
cambridge=
// Now revise the Boston weather, which was corrected
Weather bostonToday=
bostonToday= boston.
boston.getWeather();
getWeather();
bostonToday.setAvgTemp(41.0);
“Boston”
City
40.0 0.0
Weather
“Cambridge”
City
Objects As Arguments
today=
Weather today= new Weather(40.0, 0.0);
City boston=
boston= new City("Boston", today);
boston=
City cambridge=
cambridge= new City("Cambridge", today);
cambridge=
// Now revise the Boston weather, which was corrected
Weather bostonToday=
bostonToday= boston.
boston.getWeather();
getWeather(); bostonToday=
bostonToday.setAvgTemp(41.0);
“Boston”
City
40.0 0.0
Weather
“Cambridge”
City
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Objects As Arguments
today=
Weather today= new Weather(40.0, 0.0);
City boston=
boston= new City("Boston", today);
boston=
City cambridge=
cambridge= new City("Cambridge", today);
cambridge=
// Now revise the Boston weather, which was corrected
Weather bostonToday=
bostonToday= boston.
boston.getWeather();
getWeather(); bostonToday=
bostonToday.setAvgTemp(41.0);
“Boston”
City
41.0
40.0 0.0
Weather
“Cambridge”
City
When objects are passed as arguments to methods, the method
makes a copy of the reference to the object, not a copy of the object!
Why?
Method Calls With Objects
• When passing object references as arguments to
a method:
– The method makes its own copy of the references
– It makes changes to the objects through its local copies
of the references
– No changes can be made to the references (arguments)
• The method can’t change the reference to another object,
for example
– Results are returned through the return value, which
may be an object
• When passing built-in data types as arguments to
a method:
–
–
–
–
The method makes its own copy of the built-in variables
It makes changes to its local copies only
No changes are made to the arguments
Results are returned through the return value
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If you don’t like this…
• When you pass an object reference as an
argument to a method, the method may make its
own local copy of the object:
public class SaferCity {
private String name;
private Weather cityWeather;
cityWeather;
}
public SaferCity(String
SaferCity(String n, Weather c) {
name= n;
double temp= c.getAvgTemperature
c.getAvgTemperature();
getAvgTemperature();
double prec=
prec= c.getPrecipAmt
c.getPrecipAmt();
getPrecipAmt();
cityWeather=
cityWeather= new Weather(temp, prec);
prec);
}
public String getName()
getName() {
return name;
}
public Weather getWeather()
getWeather() {
return cityWeather;
cityWeather;
}
// Weather must have methods getAvgTemperature(),
getAvgTemperature(), getPrecipAmt()
getPrecipAmt()
Access: Variables, Methods
• Instance and static variables and methods have 4
access modifiers:
– Private: Access only to own class’ methods
• Data fields should be private, almost always
• Other objects of same class can access private variables
– Public: Access to all methods, all classes
• Methods intended for other class’ use are public
• Methods for internal use only are private
– Package: Access to methods of classes in same package (a
package is a group of classes)
• This is the default, alas. Always specify scope explicitly
• No ‘package’ keyword; it’s the default with no keyword
– Protected: Used with inheritance (covered later)
• Like a private variable, except it’s visible to derived or
subclasses (and, in Java, to other classes in package)
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Packages in Eclipse
In Eclipse:
File -> New -> Package. Type ‘weather’
Use lower case names by convention
Create a new class Observation in weather
(File -> New -> Class … )
Class Observation
package weather;
// Eclipse wrote this for you
// Cut and paste this from the download, or import it
public class Observation {
private double humidity;
private double cloudCover;
cloudCover;
public Observation(double h, double c) {
humidity= h;
cloudCover=
cloudCover= c;
}
public double getHumidity()
getHumidity() {
return humidity;
}
public double getCloudCover()
getCloudCover() {
return cloudCover;
cloudCover;
}
public String toString()
toString() {
return ("Humidity: "+ humidity+
" ; Cloud cover: "+cloudCover
"+cloudCover);
cloudCover);
}
}
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Add Observation to City
• In your default package in Lecture 11:
– Modify your City class to also have an
Observation object:
•
•
•
•
Add import weather.*; on 1st line of City.java
Add a private Observation obj
Modify your constructor
Add a getObservation method
• We’ll show the solution on the next slide,
and then go on to modify WeatherTest to
use your new City and Observation
Modify WeatherTest
• Change WeatherTest, still in the default
package, to:
–
–
–
–
Create a new Observation
Place it in Boston and Cambridge
Output it (System.out.println) for Boston
Remember to import weather.*; on line 1
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Package access
• If we added another class AdvancedObservation
to package weather
• And we made humidity and cloudCover package
access variables by removing the private
keyword (in an Observation2 class)
– We also remove the getXXX methods as unneeded
• Then AdvancedObservation can use Observation
data members, such as humidity and cloudCover
directly. It can simply say, for an Observation2
object obs:
– obs.humidity, or obs.cloudCover
as if they were in the AdvancedObservation class
Modified Class Observation
package weather;
public class Observation2 {
double humidity;
// No keyword means package access
double cloudCover;
cloudCover; // No keyword means package access
public Observation2(double h, double c) {
humidity= h;
cloudCover=
cloudCover= c;
}
}
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Class AdvancedObservation
package weather;
public class AdvancedObservation {
double dewpoint;
// Package access
dewpoint;
Observation2 obs;
// Package access
obs;
public AdvancedObservation(double
AdvancedObservation(double d, Observation2 o) {
dewpoint=
dewpoint= d;
obs=
obs= o;
}
public String toString(){
toString(){
return ("Humidity: "+obs
"+obs.humidity+"
obs.humidity+" ; Cloud cover: " +
obs.
obs.cloudCover+
cloudCover+ " ; Dewpoint:
Dewpoint: "+ dewpoint);
dewpoint);
// We can use obs.XXX
obs.XXX directly
// Observation2 could use dewpoint directly also
}
}
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